The field of tribology involves the study of friction and wear on materials. In the course of study two or more objects are brought into contact with each other, and a relative motion is started between the two contacting materials for the purpose of measuring the resulting friction forces. Rubbing the two materials against each other results in a damage or wear of the tested objects, and over time a wear track may be created on one or both of the objects; therefore, the wear on one or both of the objects can also be measured.
There exist many different configurations of mechanical testers, each performing a specific dedicated test.
The conventional equipment used for measuring friction and wear is dedicated to a particular test type and a corresponding test configuration. Examples include, without limitation, the configurations referred to in the art as block-on-ring, pin/ball/disk-on-disk, and reciprocating pin/ball on flat. The first represents a configuration where the sample stage includes a horizontal drive shaft rotating around its main axis. A ring specimen is coupled to the shaft for concurrent rotation, and a test block is pushed radially against the edge of the ring with a known force. A friction force and/or a torque imparted on the shaft are measured, from which a coefficient of friction between the block and the specimen material can be calculated based on the known load (i.e., a normal force applied from the block).
Similarly, for the ball-on-disk, pin-on-disk or disk-on-disk test configurations, a disk specimen is mounted horizontally on a vertical rotating shaft in the sample stage. A ball or pin specimen is brought down from above into contact with the spinning face of the disk at a known radial distance from the axis of the shaft, and a known normal force is applied. Thus, a frictional force between the ball or pin and the spinning disk and the resulting wear can be measured. Alternatively, a fixed disk, rather than a ball or pin, is aligned axially with the spinning disk coupled to the stage, and the two are brought into contact with a known force. In this configuration, the friction and wear between the two disks can similarly be measured.
The third exemplary type of test equipment is a reciprocating-type tester. In this configuration, an eccentric crank is used to transfer a rotary motion of the drive shaft to a reciprocating motion in a horizontal plane of the stage where the sample is mounted. By applying a rotational motion to the drive shaft, the horizontal reciprocating motion follows a sinusoidal velocity profile. A test specimen (a flat sample) is mounted on the reciprocating plane, and again a ball or pin specimen is brought into contact with a known normal force. The resulting frictional force is measured, and the coefficient of friction can be calculated. Wear tests can be similarly carried out in a conventional manner.
The fourth exemplary type of test equipment is a fretting tester, which is a variant of a reciprocating-type tester. In this configuration, a linear electro-magnetic actuator is used to enforce a reciprocating motion of the stage where a sample is mounted, usually at a high frequency, with a short amplitude, and in a horizontal plane. By applying a sinusoidal current to the actuator, the horizontal reciprocating motion follows a sinusoidal velocity profile. A test specimen (a flat sample) is mounted on the reciprocating stage, and again a ball or pin specimen is brought into contact with a known normal force. The resulting frictional force is measured, and the coefficient of friction can be calculated. Wear tests can be similarly carried out in a conventional manner.
Each of these tests is normally carried out on a specific, dedicated test machine. Therefore, multiple machines are required, and this is inefficient and therefore undesirable, especially if all available machines are not used simultaneously. In other words, a large area of a laboratory space, which is typically the most expensive space in a production facility, most time will remain unused. Furthermore, specialized test units will incorporate their own dedicated computers, controllers and sensors, the use of which could not be shared with other units of test equipment.
One example of a friction tester is disclosed in U.S. Pat. No. 6,615,640 issued to H. Ahn on Sep. 9, 2003 and relates to a fine friction and wear test apparatus for a plate specimen. The apparatus comprises a fixing unit, a driving unit installed on the fixing unit for fixing and moving a certain plate specimen, a ball specimen support member for rubbing and wearing the plate specimen, a rotation plate position controller fixed on the fixing unit for controlling the position of the ball specimen support member, a ball specimen controller for controlling the ball specimen support member, a controller for detecting the degree of a friction and wear of the plate specimen and controlling each mechanical and circuit part, and a power supply unit for supplying a power to a part which requires a certain power.
U.S. Pat. No. 7,013,706 issued on Mar. 21, 2006 to R. Tarumi discloses a friction force measurement apparatus which measures a friction force between a fixed member fixed on a main body of a magnetic tape drive and a magnetic tape abrading the fixed member is characterized by being equipped with a vibration detector which is joined with the fixed member and a vicinity of the fixed member and detects vibration in abrasion of the magnetic tape with the fixed member, and a calculation device which calculates the friction force between the fixed member and the magnetic tape based on a signal from the vibration detector.
U.S. Pat. No. 6,430,520 issued on Aug. 6, 2002 to M. Tranquilla discloses a dynamic friction measurement apparatus, which includes a load cell, accelerometer, and a computational device for determining the coefficient of friction corrected for inertial forces which otherwise cause an error in the calculation. The calculation device has the functions of simultaneously receiving the signals, conditioning the signals, creating output in digital or analog electrical signals, and storing or providing a value for the coefficient of friction from the dynamic measurements. A method for detecting and processing the coefficient of friction during dynamic condition is also provided.
U.S. Pat. No. 5,736,630 issued on Apr. 7, 1998 to J. Weiner discloses a compact and portable apparatus directed toward measuring the coefficients of both static and sliding friction or slip resistance occurring between two surfaces. Means are provided for determining and recording data to establish such friction accurately, repeatably, and in a form suitable for computer entry and data processing. Improvement over the prior art is provided with respect to mechanical configuration, ease of use, plus the acquisition and analysis of data, particularly for conditions involving wet or damp surfaces. A method is disclosed for essentially automatic determination of coefficient of friction.
Attempts have been made to provide a universal material tester by incorporating replaceable modular sample stages. For example, U.S. Pat. No. 9,752,969 issued on Sep. 5, 2017 to D. Werner et al. describes a universal tester (FIG. 1), wherein alternative modular sample stages are available for use in different test configurations. This tester, which in general is designated by reference numeral 10, has a frame 11, a carriage 12 moveable in a vertical direction indicated by arrow Z, a slide 13 moveable in a horizontal direction indicated by arrow X, a force sensor 14 attached to the slide, a holder 15 for an upper specimen 16, attached to the force sensor, and a base 17 attached to the frame. Each modular sample stage 18, which is attachable to the base in the frame of the unit, includes a support for the lower specimen and a mechanism for moving the lower specimen. The base includes a motor (not shown) permanently attached to the frame and a set of connectors wired to a processor (not shown) that controls the tester operation. Each of the alternative modular sample stages 18 has an adapter, which engages with the shaft of the motor included in the base when the sample stage is attached to the base. The adapter connects the motor shaft with the mechanism for producing a motion of the lower specimen supported by the sample stage. Also, each of the alternative modular sample stages has a set of connectors mating with the connectors in the base and coupled with an identification device mounted in the sample stage. When the sample stage is attached to the base the identification device connects through the mating connectors in the sample stage and in the base to the processor controlling the tester operation. The identification device automatically enables the processor to execute a subset of test operations corresponding to the attached sample stage.
Although this type of test system requires less laboratory space and can be more economically efficient than multiple dedicated testers, nevertheless it has a number of disadvantages, namely: 1) each replaceable alternative modular sample stage is driven by the same common motor, and this cannot provide speed and torque in range that could satisfy optimal conditions for all possible applications; 2) the force sensor assembly with the upper specimen holder is attached to the slide mounted on the vertically movable carriage in such a manner that creates a significant leverage of the force acting on the carriage loading mechanism and causes an unwanted parasitic moment in this mechanism; 3) each force sensor requires a manual software set-up and a system configuration corresponding to the sensor type and range or there must be a dedicated piece of software for each force sensor type and range; 4) the provision of separate environment chambers for measuring friction and wear characteristics of materials at various environmental conditions makes it difficult to set control parameters since such a setting is carried out manually and easily may lead to an error since this setting requires to take into account various control parameters which depend on the chamber volume, working range, etc., and must be individually defined for each chamber model.